The present invention relates to a dispersion liquid of silica particles for polishing with the excellent stability in dispersion, a method of producing the same, and a polishing agent comprising the dispersion liquid of silica particles.
In a production process of a substrate with a semiconductor integrated circuit provided thereon, as irregularities or steps are generated, for instance, when the circuit is formed on a silicon wafer with metal such as copper, the metallic portion of the circuit is preferentially removed by polishing to eliminate the irregularities or steps on the surface thereof. When aluminum wiring is formed on a silicon wafer and such an oxide film as SiO2 film is provided as an insulating film thereon, also irregularities are generated due to the wiring, so that the oxide film is polished for flattening.
As one of the polishing methods available for the purpose described above, there has been well known the chemical mechanical polishing method. In this method, a substrate with irregularities thereon is pressed to a rotating polishing pad and also the substrate itself is dipped into a slurry of a polishing agent, when particles for polishing included in the slurry is pressed to the substrate with irregularities thereon and as a result convex metallic sections on the substrate are removed and the surface is flattened. Further polishing in the same manner is performed also for flattening a surface of an oxide film formed on a substrate or for eliminating irregularities on an insulating film (oxide film) formed on a circuit.
In this polishing step, spherical particles each with the average particle diameter of about 200 nm such as those of fumed alumina or fumed silica are used as the particles for polishing. As the polishing agent, together with the particles for polishing as described above, oxidants such as hydrogen peroxide for improving the metal polishing speed, benzotriazole (BTA) for suppressing corrosion or oxidation of metal, or water-based slurry for polishing with a chemical polishing agent such as an acid added therein are used according to a type of the metal to be processed.
In the process for polishing a substrate as described above, it is required that a surface after having been subjected to polishing is a flat one without any irregularity or microscopic flaw, and further the polishing speed should be high. In addition, in the field of semiconductor materials, a degree of circuit integrated has been becoming higher and higher in association with the tendency for size reduction and more sophisticated performance of electric and electronic products, but sometimes the intended performances can not be achieved or troubles may occur due to impurities remaining in the transistor separation layers. Especially when any alkali metal, especially sodium is deposited on a surface of a polished semiconductor substrate or a polished oxide film, the dispersibility is very high, and the alkali metal is trapped in defective sections in the oxide film, and even if a circuit is formed on the semiconductor substrate, sometimes the circuit may be shorted, or the dielectric performance may become lower. Therefore the troubles as described above may occur under some conditions for use thereof or when the semiconductor substrate is used for a long time.
The present invention was made to solve the problems described above, and it is an object to provide a dispersion liquid of silica particles for polishing with a low content of Na ions and also with a specific range of content of ions other than Na ions, a method of producing the dispersion liquid, and a polishing agent comprising the dispersion liquid.
The present invention provides a dispersion liquid of silica particles for polishing with an average particle diameter in the range from 5 to 300 nm, and Na ion content in the silica particle is less than 100 ppm, while content of ions other than Na ions is in the range from 300 ppm to 2 weight %.
The method of producing the dispersion liquid of silica particles for polishing according to the present invention comprises the steps of adding an acidic silicic acid solution into a dispersion liquid of core particles and growing the core particles to obtain a dispersion liquid of silica particles, and the steps are performed to grow the particles under the conditions that a content of Na ions (Ns) defined by a number of Na ions per square meter of the core particle is less than 50xc3x971017 pieces/core particle""s surface area(m2), that a content of ions (Nk) other than Na ions defined by a number of ions other than Na ions per square meter of the core particle is in the range from 5xc3x971017 to 2000xc3x971017 pieces/core particle""s surface area(m2), and that the ratio of (Nk)/(Ns) is 5 or more.
An alkali silicate should preferably be added to the dispersion liquid of core particles before the acidic silicic acid liquid is added thereto.
An polishing agent according to the present invention comprises the dispersion liquid of silica particles.
Preferable embodiments of the present invention are described below.
1. Dispersion Liquid of Silica Particles for Polishing
A content of Na ions in a silica particle in the dispersion liquid of silica particles for polishing according to the present invention, when calculated as a content of Na in SiO2, is less than 100 ppm, preferably less than 50 ppm, and more preferably less than 20 ppm.
If the Na ion content in a silica particle exceeds 100 ppm, Na remains on a substrate to be polished when the particles are used as a polishing agent, and the residual Na may cause insulation fault or short circuitry in a circuit formed on the semiconductor substrate, which may in turn lower the dielectric constant of a film (insulating film) provided for insulation, and such troubles as increase in impedance, a slower response speed, and increase of power consumption may occur. Further the Na ions may move (disperse) and cause the troubles as described under some specific conditions or when used for a long time.
The silica particle includes ions other than Na ion such as monovalent cations of Li, K, Rb, Cs, NH4 and others, and bivalent cations of Mg, Ca, Sr, Ba and others. A content of these ions other than those of Na is in the range from 300 ppm to 2 weight %, and preferably in the range from 300 ppm to 1 weight %.
When the content of ions other than Na ions is less than 300 ppm, a quantity of cations on a surface of the silica particle is too small and stability of the silica particle dispersion liquid is poor, which gives some negative effects for such use as the low workability with low cost performance, and even if it is used as a polishing agent or a polishing material, sometimes a sufficient polishing speed can not be obtained. When the content of ions other than Na ions is over 2 weight %, many ions other than Na ions remain on a surface of a substrate and/or an insulating film, and the troubles as described above may occur when it is used for a long time. Of the ions other than Na ions, cations of K, Rb, Cs, and NH4 each having a larger ion diameter as compared to that of the Na ion are preferable because the ions seldom cause such troubles including insulation fault as described above.
The average particle diameter of the silica particles described above varies according to such factors as the required polishing speed and polishing precision, but it should preferably be in the range from 5 to 300 nm, and more preferably in the range from 10 to 200 nm. When the average particle diameter is less than 5 nm, the silica particle dispersion liquid will generally lack the stability, and further the particle diameter is too small to obtain a sufficient polishing speed. On the other hand, when the average particle diameter exceeds 300 nm, scratches may remain on surfaces of some specific types of substrates or insulating films, and it is impossible to obtain sufficient surface smoothness.
In the dispersion liquid of silica particles for polishing according to the present invention, the silica particles are generally dispersed in water and/or an organic solvent. The organic solvent used for this purpose is preferably such alcohols as methyl alcohol, ethyl alcohol, and isopropyl alcohol, and in addition such water-soluble organic solvents as ethers, esters, and ketone may be used in the present invention.
The concentration of silica particle dispersion liquid as described above, when calculated as that of SiO2 (solid phase), should preferably be in the range from 5 to 50 weight %. When the concentration is less than 5 weight %, the concentration of silica particles in a polishing agent is too low to obtain a sufficient polishing speed, and on the other hand, when the concentration is over 50 weight %, the stability of the dispersion liquid will become lower, and in addition dry materials will be generated and deposited on a surface of a substrate during the process of supplying the dispersion liquid for polishing, which may cause generation of scratches.
2. Method of Producing the Dispersion Liquid of Silica Particles for Polishing
Any known method may be employed as a method of producing the dispersion liquid of silica particles for polishing so far as it is possible to obtain silica particles capable of being used as a polishing agent and also having contents of Na ions and ions other than Na ions in the ranges described above respectively. For instance, in the known method of producing a sol, a solution obtained by solving silica in potassium silicate (potash water glass) with a low Na content, or in such organic base as quarternary amine may be used as a raw material in place of the sodium silicate (sodium water glass) which is generally used as the raw material. Further the silica particles can be obtained by removing Na with such a material as an acid from silica particles produced by any known method.
With any of the methods described above, however, there are some restrictions over the raw materials used for production or over the production process, and in addition the cost performance is disadvantageously low. Also with the known production methods described above, especially in a case of large silica particles with the average particle diameter of 30 nm or more, sometimes it is impossible to stably obtain silica particles with the Na ion content of 100 ppm or less.
To overcome the problems as described above, in the method of producing the dispersion liquid of silica particles for polishing according to the present invention, in a process of adding an acidic silicic acid solution to a dispersion liquid of core particles for growing the core particles in the size to obtain a silica particle dispersion liquid, a content of sodium ions and that of other ions are controlled into specific ranges respectively for growing the core particles to obtain a desired silica particle dispersion liquid.
There is no specific restriction over the core particle dispersion liquid to be used in the present invention so long as the particles function as core particle, and a dispersion liquid of fine particles such as those of silica, alumina, zirconia, and silica alumina may be used for this purpose.
Especially sol of silica and sols of silica-based composite oxide as disclosed in Japanese Patent Laid-Open Publication No. HEI 5-132309 and Japanese Patent Laid-Open No. HEI 7-105522 are preferable because distribution of particle diameters is homogenous and silica particles for polishing with homogenous distribution of particle diameters can be obtained.
It is preferable to add an alkali silicate to the core particle dispersion liquid before an acidic silicic acid solution is added therein. When the alkali silicate is added beforehand, concentration of SiO2 dissolved in the dispersant is set to a high level before the acidic silicic acid solution for particle growth is added, and therefore precipitation of silicic acid onto the core particles occurs quickly, and further pH of the dispersion liquid can easily be controlled in the range from 8 to 12, and more preferably in the range from 9.5 to 11.5. As the alkali silicate to be used in this process, it is preferable to use an alkali silicate other than sodium silicate (sodium water glass) such as potassium silicate (potash water glass) or a solution obtained by dissolving silica in such an organic base as a quarternary amine.
Further an alkali metal hydride other than NaOH, ammonium, and a quarternary ammonium hydride may be added therein according to the necessity. Further Mg(OH)2 and such alkaline earth metal hydride as Ca(OH)2, Sr(OH)2, and Ba(OH)2 may advantageously be used.
In the present invention, even if core particles are not dispersed beforehand, as the acidic silicic acid solution described later is added to an aqueous solution of alkali silicate, core particles are generated when silica concentration becomes sufficiently high, so that the core particle dispersion liquids as described above can advantageously be used.
The concentration of the core particle dispersion liquid varies according to a size of the core particles, but the concentration as calculated as that of SiO2 should preferably be in the range from 0.005 to 20 weight %, and more preferably in the range from 0.01 to 10 weight %. If the concentration of core particle is less than 0.005 weight %, a portion or all of the core particles may be dissolved when the temperature for particle growth is made higher, and if all core particles are dissolved, the effect of using the core particle dispersion liquid can not be achieved. When a portion of the core particles is dissolved, diameters of the obtained particles are generally not homogenous, which sometimes makes it impossible to achieve the effect of using the core particle dispersion liquid. On the other hand, when the concentration of core particles exceeds 20 weight %, the speed of adding a silicic acid solution must be made higher to adjust an addition rate of the acidic silicic acid solution per core particle to the same level in a case of low concentration, but in this case, precipitation of the acidic silicic acid solution onto a surface of the core particle is too slow, and the acidic silicic acid solution may be gelatinized.
There is not specific restriction over an average particle diameter of the core particles so far as the desirable silica particles as described above can be obtained, but generally the average particle diameter should preferably be in the range from 2 to 280 rim, and further in the range from 3 to 100 nm. When the average particle diameter of core particles is less than 2 nm, although the circumstances vary according to a temperature or a concentration of the dispersion liquid as well as to a quantity of alkali, a portion or all of the core particles may be dissolved when the temperature is made higher for growing the core particles, and any of the troubles as described above may occur. Silica particles can be obtained even if the average particle diameter is less than 2 nm, but distribution of the particle diameters are apt to become inhomogeneous. On the other hand, as silica particles with the average particle diameter of 300 nm or more are not desired for producing the dispersion liquid according to the present invention, those with the average particle diameter of 280 nm or more are not preferable.
In the method according to the present invention, a content of Na ions (Ns) in the core particle dispersion liquid described above as defined by a number of Na ions per square meter of a surface area of the core particle is preferably less than 50xc3x971017 pieces/core particle""s surface area(m2), and more preferably less than 20xc3x971017 pieces/core particle""s surface area(m2). When the Na ion content (Ns) is more than 50xc3x971017 pieces/core particle""s surface area(m2), even if there are many ions other than Na ions, the content of Na ions in the particles finally obtained is not less than 100 ppm, and the particles are not adapted to be used for polishing.
As for the content of ions other than Na ions (Nk) in the core particle dispersion liquid as defined by a number of ions other than Na ions in the core particle dispersion liquid per square meter of a surface area of the core particle should preferably be in the range from 5xc3x971017 to 2000xc3x971017 pieces/core particle""s surface area(m2). When the content of ions other than Na ions (Nk) is less than 5xc3x971017 pieces/core particle""s surface area(m2), as the content of ions other than Na ions is small, a quantity of ions other than Na ions existing on a surface of the core particle corresponding to OH bases thereon is small, so that, in the state where Na ions have been carried in and deposited from the acidic silicic acid solution described below on a surface of the core particle, the silicic acid is precipitated thereon to cause growth of core particles, and as a result sometimes the content of Na ions in the obtained silica particle may be over 100 ppm, which is not preferable. On the other hand, when the content of ions other than Na ions (Nk) is over 2000xc3x971017 pieces/core particle""s surface area (m2), a quantity of ions other than Na ions is more than that required for covering a surface of the core particle, and therefore the effect of suppressing in-take of Na ions as described above is not further enhanced, nor is further reduced a quantity of Na ions in the obtained silica particles. Further, the content of residual ions other than Na ions may be over 2 weight %, which is not preferable.
A ratio of the content of ions other than Na ions (Nk) to a ratio of Na ions (Ns), namely (Nk)/(Ns) is required to be more than 5. If the (Nk)/(Ns) ratio is less than 5, the Na ions existing corresponding to OH bases on a surface of the core particle are not sufficiently substituted by other ions, and as a result sometimes the content of Na ions in the obtained silica particle may be more than 100 ppm.
In the production method according to the present invention, a temperature of the core particle dispersion liquid is kept in the range from 50 to 200xc2x0 C., more preferably in the range from 60 to 150xc2x0 C., and the core particles are grown by continuously or intermittently adding the acidic silicic acid solution therein.
The acidic silicic acid solution can be obtained by removing Na ions with an ion exchange resin from a sodium silicate aqueous solution with the SiO2 concentration in the range from 1 to 10 weight %, and more preferably in the range from 2 to 7 weight %. The SiO2 concentration also in the acidic silicic acid solution is generally in the range from 1 to 10 weight %, and more preferably in the range from 2 to 7 weight %. It should be noted that use of such a material as potassium silicate is preferable, because in that case Na ions do not substantially exist in the acidic silicic acid solution and a content of Na in the obtained silica particle can be reduced.
A rate of adding the acidic silicic acid solution varies according to an average particle diameter of core particles or a concentration of core particles in the dispersion liquid, and should preferably be set to a level where fine particles other than core particles are not generated. Further the acidic silicic acid solution can be added in batch or repeatedly several times until silica particles having the desired average particle diameter are obtained.
After the acidic silicic acid solution is added, the mixture may be aged for about 0.5 to 5 hours under the temperature in the range from 70 to 150xc2x0 C. When the aging as described above is performed, the Na ion content in the obtained silica particle may be further reduced with the particle diameter distribution becoming more homogeneous. Further it is possible to remove excessive ions by using an ultra filtration membrane or condensing or diluting the concentration to a required level according to the necessity. In addition, it is possible to obtain a silica particle dispersion liquid in which the aqueous solvent is substituted with an organic solvent by means of the ultra filtration membrane method or distillation method.
3. Polishing Agent
The polishing agent according to the present invention contains the dispersion liquid of silica particles for polishing. Generally water is used as a solvent for this dispersion liquid, but such alcohols as methyl alcohol, ethyl alcohol, isopropyl alcohol, or such water-soluble organic solvents as ethers, esters, and ketones may be used according to the necessity.
The concentration of silica particles for polishing in the polishing agent should preferably be in the range from 5 to 50 weight %, and more preferably in the range from 5 to 30 weight %. When the particle concentration is less than 5 weight %, the concentration is too low for some types of substrates or insulating films, so that the polishing speed is low and the productivity is low, and when the particle concentration is more than 50 weight %, stability of the polishing agent is not sufficient with the polishing speed or polishing efficiency not further improved, and further dry materials may be deposited in the step of supplying the dispersion liquid for the polishing processing, which may in turn generate scratches.
Hydrogen peroxide, acetate peroxide, urea peroxide, or a mixture thereof may be added to the polishing agent according to the polishing agent, if required. When the material to be polished is metal, it is expected that the polishing speed can be improved by adding the hydrogen peroxide or others.
Such organic acids as phthalic acid or citric acid or salts of these acids may be added as a complex forming agent to the polishing agent according to the present invention. Further for suppressing oxidation, such a material as benzotrianole (BTA) may be used. Further pH of the polishing agent can be adjusted by using an acid or a base. A surfactant or the like may be added for improving dispersibility or stability of the polishing agent.
The dispersion liquid of silica particles for polishing according to the present invention has a prespecified quantity of ions other than Na ions, so that the stability is quite excellent.
In the polishing agent according to the present invention, as a content of Na ions in the silica particle is low, so that Na does not remain on a surface of a polished semiconductor substrate or other processed work, and a high polishing speed can be obtained. Further a surface of the substrate is made smooth after polishing, and insulation fault or short circuitry due to residual Na never occurs. Further in the semiconductor substrate or other materials to be processed, a dielectric factor of the insulating film becomes lower with the impedance in the metal wiring increased, so that such troubles as lowering of the response speed and increase in power consumption never occur, and further any troubles due to movement of residual Na ions do not occur under any condition for use, or even if it is used for a long time.